Assessing and managing the risk of earthquake-induced liquefaction to civil infrastructure - Seismic Risk Analysis and Management of Civil Infrastructure Systems - page 131

• Analytical methods . For a detailed analysis, an analytical approach

which considers both the potential ground deformation resulting from
liquefaction, and the response of a specifi c structure or lifeline to the
demand imposed by the ground deformation, can be used. For specifi c
vulnerable items, an analytical approach is appropriate. However, as
always, the less information is available, the more uncertain the analyti-
cal results will be, and a balance is required between the requirements
of the analysis in terms of cost, time and resources, and the reliability
of the output.

• Fragility curves . Fragility curves, mainly used to assess the potential for

damage to existing infrastructure (for example, in earthquake loss esti-
mation for insurance or disaster management purposes), defi ne the
probability of a particular type of structure being in a given damage
state as a result of a defi ned level of demand (a function of earthquake
hazard). They can be derived from analysis, expert opinion, or case
history data, or often a combination of all three. Many published fragil-
ity curves exist for the response of different building types to grounds
shaking. However, there are very few curves presently available for the
response of buildings, bridges, roads or pipelines to liquefaction-induced
permanent ground deformation. The HAZUS tool developed by the
Federal Emergency Management Agency (FEMA, 2003) to estimate
losses resulting from a number of natural hazards including earthquakes,
presents fragility curves for permanent ground deformation. However,
the simplifi cations involved in terms of the assessment of the expected
PGD and of the infrastructure response mean that these types of curves
are only appropriate for large portfolio analyses, not for individual
structures or lifelines.

5.5 Tolerable risks and performance levels

For a risk assessment, it is important to clarify the risk metric . There are
many units and scales that can be used to defi ne risks, including:

• costs (direct or indirect);

• time (impact on programme and down-time during operation);

• safety (to workers and the general public);

• less tangible measures such as reputational damage to an owner/opera-

tor, or societal impact.

Damage related to ground deformation has the potential to cause all of
the above types of loss. For a risk-based approach as presented in this
chapter, it is important to assess and communicate risks in a consistent
format and time frame to the overall risk management methodology for
the project.